The present invention relates to a method for operating an internal combustion engine.
By way of example, DE 195 19 663 A1 has disclosed a method for operating an internal combustion engine with spontaneous ignition, in which in a first stage a homogenous, precompressed fuel/air mix which is not suitable for spontaneous ignition is provided in the working space, and in a second stage an additional quantity of the same fuel is injected into the working space in order to bring about the spontaneous ignition. In this case, the fuel/air mix is prepared by external mix formation and introduced into the working space, where it is compressed to close to the spontaneous ignition point. The injection of the additional quantity of fuel in the second stage takes place in finely atomized form, avoiding contact with the walls, so as to form a mix cloud in which, on the one hand, the fuel/air ratio is no greater than the stoichiometric mixing ratio and in which, on the other hand, the spontaneous ignition condition is achieved.
Furthermore, DE 198 52 552 C2 has disclosed a method for operating a four-stroke internal combustion engine which, at part-load, forms a lean base mix from air, fuel and retained exhaust gas and, at full load, forms a stoichiometric mix. At part load, compression ignition takes place, whereas at full load spark ignition takes place. Furthermore, mechanically controlled exhaust-gas retention with switchable valve closure overlap and exhaust gas throttling is provided. An activation injection can be carried out into the retained exhaust gas. The quantity of retained exhaust gas, with the valve closure overlap switched on, is controlled or preset as a function of the engine speed and engine load by an exhaust-gas throttle valve which is active for all the combustion chambers. The pressure when the intake members open into the individual combustion chambers is evened out by a cylinder-selective, cylinder-consistent activation injection.
A method for operating a four-stroke, reciprocating-piston internal combustion engine is also known from DE 198 18 569 C2. It is characterized by a homogenous, lean base mix of air, fuel and retained exhaust gas and by compression ignition and direct injection of the fuel into the combustion chamber. The volume of the combustion chamber changes cyclically. The combustion chamber can be filled with fresh gas through at least one intake member, while the combustion exhaust gases can be at least partially expelled through at least one exhaust member. In the part-load range and in the lower full-load range, the internal combustion engine is operated with compression ignition and preferably mechanically controlled exhaust-gas retention, whereas in the full-load range and high part-load range it is operated on the Otto cycle.
One drawback of the above known methods documents is in particular that the temperature of the exhaust gas and the composition of the working gas change in the event of changes in load. The reactivity of the mix during compression ignition is likewise altered as a result, even to the extent of causing misfires if the working gas temperatures are too low.
By contrast, an object of the invention is to provide a method for operating an internal combustion engine which takes into account and/or corrects changes in the reactivity of the mix in the event of changes in engine speed.
This object is achieved by a method whereby in the event of a change in the load state, the theoretical shift in the combustion position is compensated for by the shift in the phase position of intake and/or exhaust phase. A targeted change in the control times of this nature allows changes in the working gas composition in the event of load changes to be effectively corrected.
In the event of a reduction in the engine load, either the intake phase can be shifted in the late direction or the exhaust phase can be shifted in the early direction, or the two phase shifts can be carried out simultaneously, in which case the effects are cumulative.